NASA for middle-school students http://www.nasa.gov/audience/forstudents/5-8/features/index.html
National Geographic for Kids http://www.nationalgeographic.com/kids/
MathCounts: Problem of the week and other great math links http://www.mathcounts.org
FunBrain: Fun math practice! http://www.funbrain.com
Gamequarium: Fun math practice! http://www.gamequarium.com/prealgebra.html